What are the characteristics of viruses?

Viruses are obligate intracellular parasites of nucleic acid and protein. Their RNA or DNA genome is covered with a protein coat. They are among the smallest infectious agents known. They range in size from the tiny polio virus (2 nm diameter) to the large pox virus, which is 400 nm in diameter.

Viruses are also looked upon as highly evolved mobile genetic elements that have cellular origin. Viruses exhibit typical morphological and physical characters including a great variety in size and shape, chemical composition, structure of the genome and mode of replication.

Viruses lack most of the enzymes necessary for the metabolism and synthesis of complex molecules and so can replicate only inside a living cell.

The strandedness of genome of a virus is a characteristic feature. The viral genome may consist of one type of nucleic acid either DNA or RNA which may be single stranded or double stranded. Viruses make us of the ribosomes of their host cells; this defined as absolute parasitism.
What are the characteristics of viruses?

Measles

A childhood viral illness causing a widespread red rash and fever considered to be the most contagious disease in the world. Widespread epidemics caused a great number of deaths in the Middle Ages.

Measles virus is a member of the genus Morbillivirus of the Paramyxoviridae family. The virus appears to be antigenically stable - there is no evidence that the viral antigens have significantly changed over time.

The medical name for measles is rubeola, It is sometimes called ‘red measles’ to distinguish it from the much milder disease known as rubella.

The measles virus is passed from person to person, like many other viruses, on the tiny droplets of moisture that people constantly breathe out and in.

Symptoms appear after an incubation period of between nine to 11 days, and the patient is infectious from shortly after the beginning of this period until up to a week after symptoms have developed.

The patient develops what appears to be a heavy cold with a husky cough. The nose is runny and the eyes are red and watery. They may a fine, red rash that last for a few hours then disappears.

The second stage of the infection starts after 12-13 days with a reddening of the palate and the mucous membrane of the cheeks. Two to three days later the typical maculopapular rash develops, starting behind the ears spreading over the whole body.
Measles

Variola virus of smallpox

Poxviruses that cause disease in humans belong to four different genera of the vertebra poxvirus subfamily. The majority these viruses belong to the genus Orthopoxvirus.

Poxviruses were first visualized by electron microscopy in 1938. The genus Orthopoxviruses comprises many related viruses and includes, in addition to variola major, vaccinia, virus, cowpox, monkeypox, ectromelia, camelpox, taterapox, Uasin Gishu disease and racoonpox.

The orthopoxvirus Variola virus is the only member species of the Orthopoxvirus genus which is a solely human pathogen.

Variola virus, like all orthopoxviruses, replicates solely in the cytoplasm of infected cells. At least two types of Variola virus have been clearly described that have distinct biological and genetic properties and different human clinical and epidemiological manifestation.

These have been described as variola major and Variola minor. Another variant has been suggested, intermedius.

The virion of Variola virus has the same size and shape as that of all other orthopoxviruses.

The most important property of variola virus, in relations its effects on the human host, is its virulence. Some epidemics of smallpox were associated with high mortality and other with a low mortality.

The genetic difference between the variola strains most likely reflects differences in their replication and host-cell assembly leading to a greater or lesser virulence.
Variola virus of smallpox